The present invention relates to the art of PBZ polymers (also known as PBX polymers, see definition hereinafter) and polymer compositions containing blocks of those polymers.
PBZ polymers, i.e. polybenzoxazole, polybenzothiazole and polybenzimidazole, and their synthesis are described in great detail in the following patents which are incorporated by reference: Wolfe et al., Liquid Crystalline Polymer Compositions and Process and Products, U.S. Pat. No. 4,703,103 (Oct. 27, 1987); Wolfe et al., Liquid Crystalline Polymer Compositions and Process and Products, U.S. Pat. No. 4,533,692 (Aug. 6, 1985); Wolfe et al., Liquid Crystalline Poly(2,6-Benzothiazole) Compositions, Process and Products, U.S. Pat. No. 4,533,724 (Aug. 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,693 (Aug. 6, 1985) (hereinafter the Wolfe '693 patent) and Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Pat. No. 4,578,432 (Mar. 25, 1986).
PBZ polymers are noted for their high strength, for their high thermal stability and for their low processability. To process PBZ polymers, the polymers are ordinarily dissolved in a mineral acid to form liquid crystal compositions which are then spun to form fibers or processed to form films.
To improve processability, rigid rod PBZ polymers can be incorporated into a molecular composites with flexible polymers. Hwang et al., "Solution Processing and Properties of Molecular Composite Fibers and Films," 23 Polymer Eng. & Sci. 784 (1983); Hwang et al., "Phase Relationships of Rigid Rod Polymer/Flexible Coil Polymer/Sovent Ternary Systems," 23 Polymer Eng. & Sci. 789 (1983); and Hwang et al., "Composites on a Molecular Level: Phase Relationships, Processing and Properties," B22 J. Macromol. Sci.-Phys. 231 (1983), which are incorporated herein by reference.
Due to the poor miscibility of PBZ polymers with thermoplastic or flexible polymers, such composites often experience phase separation and a resulting loss of strength, clarity or other desirable properties, particularly if heated above the glass transition temperature of the thermoplastic polymer. To improve the miscibility of PBZ polymers, they have been formed in block copolymers with AB-PBO, -PBT or -PBI polymers, a non-thermoplastic flexible coil variant of the PBZ polymer. Tsai, Method for Making Heterocyclic Block Copolymer, U.S. Pat. No. 4,544,713 (Oct. 1, 1985); Gordon et al., "Synthesis and Characterization of Segmented Block Copolybenzimidazoles Containing Rigid Rod and Flexible Coil Segments," 28(2) Polymer Preprints 308 (1987); Gordon et al., "Thermally Stable Block Copolymers," 26(1) Polymer Preprints 146 (1985); Tsai et al., "High Strength Modulus ABA Block Copolymers," 26(1) Polymer Preprints 144 (1985); Krause et al., "Morphology and Mechanical Properties of a Phase Separated and a Molecular Composite 30% PBT/70% ABPBI triblock copolymer", 29 Polymer 195 (1988); and Gordon et al., "Thermally Stable Block Copolymers," 27(1) Polymer Preprints 311 (1986). It is also reported that graft copolymers of PBI and polypropylene oxide have been synthesized. Evers, "Graft Copolymers of Rod-like Poly(p-phenylenebenzobisimidazole)," 29(1) Polymer Preprints 244 (1988).
What is needed are new block copolymers containing PBZ moieties and particularly those which form compositions which are thermoplastic and do not experience substantial phase separation even after being heated and shaped at their glass transition temperature.